Willi Dansgaard

Willi Dansgaard, who died on January 8 aged 88, was a Danish
palaeoclimatologist and the first to realise that the Greenland ice cap was
a frozen archive of the world's climate history.

Willi Dansgaard

7:16PM GMT 15 Feb 2011

Dansgaard's expertise was in mass spectroscopy, a technique used to elucidate the chemical structure of molecules. In the 1950s he reasoned that he should be able to estimate the earth's surface temperature by using relative abundances of two naturally-occurring isotopes of oxygen in water that falls as rain or snow – oxygen-16, and the much rarer oxygen-18. The latter is slightly heavier, and Dansgaard reasoned that, at colder temperatures, slightly more water molecules containing oxygen-18 would condense as rainfall.

To test his theory he obtained rain samples from the International Atomic Energy Agency, which collected samples from all over the world to track radiation from bomb tests. As he predicted, the isotope ratios proved an excellent predictor of temperature. In 1964 he published his findings in the journal Tellus.

To see if his findings might be confirmed in Arctic ice cores, he visited Camp Century, an American Army base in Greenland, which had been set up as an early warning base for Soviet missile attacks. Dansgaard had been chipping away at the ice cap a few hundred feet at a time, but the Army had a drill that could go down thousands of feet. Collaboration began, and before long the team had drilled several miles down to bedrock.

Subsequently Dansgaard obtained access to the Dye-3 radar station on the ice cap in South Greenland, where the first purely scientific deep ice core drilling project was carried out from 1979 by a Danish-American consortium.

Working with a Swiss colleague, Hans Oeschger, Dansgaard painstakingly pieced together a complete year-by-year reconstruction of the earth's climate going back a million years, through a handful of ice ages.

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Among other things, the ice graph confirmed that the 19th century marked the end of a great chill lasting about 600 years, reaching its climax in the 1600s. Then, southern Scandinavia was a few degrees Celsius colder than now, and in London festivals were held on the frozen River Thames.

The scientists also established that the 12th-century Icelandic sagas which accused the Viking adventurer Eric the Red of dreaming up the name "Greenland'' to attract settlers to the icy wastes were being unfair: when Eric had settled there in 985AD, the weather was warmer than when the sagas were written 200 years later.

To their surprise, the scientists also discovered cycles in which drastic climate changes were found to occur much more rapidly than anybody had imagined – over decades, rather than hundreds or thousands of years. About 15,000 years ago, for example, Greenland abruptly warmed by 16 degrees over a period of 50 years. Later studies identified at least 24 of these rapid shifts, now known as Dansgaard-Oeschger events, between 100,000 and 11,500 years ago.

At the time Dansgaard first published his findings, they did not register with scientists as an important indicator of future climate trends.

One finding was that temperatures and the levels of greenhouse gases in the ice cores move broadly in tandem with each other. Though Dansgaard avoided becoming involved in the wider political debate about global warming, this finding, along with the discovery of abrupt and devastating climatic "flips", fed into the 1992 Rio Framework Convention, which called for measures to prevent "dangerous" climate change.

Willi Dansgaard was born in Copenhagen on August 30 1922 and educated at the university there, taking a doctorate in Physics. He first travelled to Greenland in 1947 to study magnetism and was "bitten by Greenland for life".

Dansgaard later organised or participated in more than 19 expeditions to the glaciers of Norway, Greenland and Antarctica, and went on to develop ways to date gases trapped in the ice as well as to analyse acidity, dust and other influences on climate, including volcanic eruptions.

Thus, analyses of the acidity levels in ice cores have shown that a particularly large eruption produced of acidic fallout on Greenland for three years roundabout 50BC (possibly supporting accounts of a dimming of the sun after Julius Caesar was assassinated in 44BC, which is reported in the writings of Virgil and Pliny the Elder).

Two sharp peaks in acidity attributed to sulphuric acid from major volcanic eruptions, probably in the remote Arctic, occurred in 1601 and 1602, when contemporary accounts reported that the sun and moon appeared "reddish, faint and lacked brilliance".

Efforts to pinpoint the catastrophic volcanic eruption of Thera, in the Aegean Sea, that is said to have formed the basis for the Atlantis legend and perhaps contributed to the downfall of the Minoan civilisation, yielded a date of 1390BC – with an error margin of 50 years.

In 1995 Dansgaard shared the Crafoord Prize of the Royal Swedish Academy with Nicholas Shackleton, and the following year shared the Tyler Prize, the highest award in environmental science, with Oeschger and Claude Lorius.